This invention relates to a switch for digitally controlling the brightness of the electron beam of an electron beam deflection tube.
It is known to control digitally the brightness or lack of light of the electron beam by variation of the cathode potential of the deflection tube by the output signal of a switching amplifier. The amplifier in turn is controlled by a digital signal present as a square wave voltage
In order to control the brightness of an electron beam the cathode of the beam deflection tube is connected to ground by the output signal while in order to provide a dark spot a voltage of about 40 volts is required. The output signal of the switching amplifier should follow as far as possible without time delay and without distortion the digital control signal.
Previously known switching amplifiers consist of several transistor switching stages connected in series. These stages are necessary for rotating the phase of a digital control signal and for amplifying the small aplitudes of the digital control signal to the high voltage required for controlling the darkness or black area produced by the electron beam. In view of the multiplicity of switching stages, the known switching amplifiers are expensive and require much equipment.
In order to obtain even for the usual spread between transistors a predetermined switching behavior, the transistors must be overdriven.
This results in switching times up to 500 nanoseconds (ns). In order to write short signs or letters gating times of about 200 ns are necessary. Therefore, the conventional switching amplifiers cannot fulfill this demand.
The brightness of the electron beam is determined by the voltage between control grid and cathode of the tube. This voltage must be maintained constant as long as a constant brightness is required. Therefore, the output voltage of the switching amplifier must also be maintained constant even when the transistor conducts.
For the known switching amplifiers the output voltage of the conductive transistor corresponds to its residual voltage. Due to the temperature dependence of the residual voltage the output voltage cannot be stable.
If the cathode of an electron beam deflection tube is controlled with a digital control signal having a steep slope and a large amplitude a noise signal is coupled by means of the capacitance between the control grid and the cathode from the cathode to the control grid. This overcoupling results in undesired brightness variations of the electron beam.
A further disadvantage of the known arrangements consists in that the picture contours can only be produced with insufficient sharpness. This is particularly apparent during the recording of short signs or those which rapidly follow each other.
It is accordingly an object of the present invention to provide a simplified switching amplifier which avoids the drawbacks previously discussed and which permits to provide an electron beam resulting in a constant brightness.